Soft x-ray pulses of submicrosecond duration are needed to test materials and components of pulsed fusion reactions. Techniques presently employed to generate such pulses are (a) electron diode guns bombarding a heavy metal target, (b) underground fusion devices and (c) dense focus with high Z material electrode tips which erode during the pulse. Electron diode guns at the required x-ray energies of fractional MeV are very inefficient because the conversion efficiency of electron beam energy into Bremsstrahlung decreases superlinearly with the decrease of electron energy for a given target anode, a fact which is well known to the designers of flash x-ray tubes. In addition, at low electron energies of fractional MeV, the space charge of electron beam is not cancelled by relativistic effects and limits severely the maximum current density of the electron beam available at the target anode. Furthermore, the electric fields at the cathode are usually not sufficient to obtain a copious electron emission by the field effect and therefore the thermionic cathodes must be employed which intrinsically yield a much lower electron emission current density than field emitters. Present electron beam-Bremsstrahlung flash generators of minimum useful x-ray fluence therefore employ electron beams in the several MeV range. They generate x-ray flashes of spectral distribution which contains most of the photon energy in the hard x-ray spectral range. Because the x-ray penetration depth decreases superlinearly with the photon energy, the deposited x-ray energy density in test materials and components is substantially different for soft and hard x-ray flashes of identical fluence at the source. Therefore, pass-fail conclusions of tests on materials, components and devices performed with many MeV energy electron beam x-ray flash generators are not directly scalable to predict the performance under a soft x-ray flash. Underground fusion flash tests suffer from the intrinsic inability to separate by the time-of-flight method the various components of radiations and expansion waves generated during the test. Therefore, various radiation and blast wave effects cannot be readily differentiated and only the cumulative, gross effects are observed. Thus, the materials designer is handicapped in separating the individual contributions from each damaging radiation.
The plasma focus alone can also be used as a soft x-ray flash generator by altering the electrode design and configuration such as to increase the evaporation and erosion of certain portions of the electrodes. Because only the energy stored in the plasma focus can be used for soft x-ray production, the fluence of x-ray flash is limited. In addition, a full control of erosion of the electrodes cannot be achieved in this case. Therefore, the intensity and the spectral distribution of x-ray flashes varies from one firing to another.
Therefore, it is an object of this invention to overcome deficiencies and eliminate or substantially reduce problems encountered in producing soft x-rays.
Another object of this invention is to provide a x-ray generator in which the energies from an electron source and a plasma generator are combined to produce soft x-rays when the plasma has been seeded with high Z material.
Still another object of this invention is to provide a x-ray generator in which electrons from an electron beam source are guided and focused onto a hot seeded plasma produced by a plasma generator.